Microfabricated implantable wireless pressure sensor for use in biomedical applications and pressure measurement and sensor implantation methods

ABSTRACT

A variable capacitor, a microfabricated implantable pressure sensor including a variable capacitor and an inductor, and related pressure measurement and implantation methods. The inductor may have a fixed or variable inductance. A variable capacitor and pressure sensors include a flexible member that is disposed on a substrate and defines a chamber. Capacitor elements extend indirectly from the flexible member. Sufficient fluidic pressure applied to an exterior surface of the flexible member causes the flexible member to move or deform, thus causing the capacitance and/or inductance to change. Resulting changes in resonant frequency or impedance can be detected to determine pressure, e.g., intraocular pressure.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of co-pending U.S. application Ser.No. 11/847,262, filed Aug. 29, 2007, which claims priority of U.S.Provisional Application No. 60/841,113, filed on Aug. 29, 2006, thecontents of both which are incorporated herein by reference as thoughset forth in full.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

The U.S. Government has certain rights in this invention pursuant GrantNo. EEC-0310723, awarded by the National Science Foundation.

FIELD OF THE INVENTION

The field of the invention relates to pressure sensors and, moreparticularly, to microfabricated implantable pressure sensors for use inbiomedical applications including monitoring of intraocular pressure.

BACKGROUND

Pressure sensor devices have been used to study various physiologicalconditions in biomedical applications. One known application is tomonitor intraocular pressure, for example, in connection with treatmentof glaucoma. Glaucoma is a well known ocular disease that affectsmillions of people. Persons afflicted with this disease requiretreatment for life. The disease causes visual field loss and if leftuntreated, may result in permanent loss of vision, and is a primarycause of blindness in the United States and elsewhere.

The exact cause of glaucoma is not known, but it is characterized bypathological changes in the optic disc and nerve fiber of the retina.Studies suggest that development of the disease may be attributable tovarious factors including elevated intraocular pressure. Normalintraocular pressure typically ranges from about 10 to about 21 mm Hg,e.g., about 15 mm Hg. Intraocular pressures of eyes of patients havingglaucoma often exceed 21 mm, although glaucoma may exist whenintraocular pressures are at lower levels. Elevated intraocularpressures are believed to be responsible for slowly damaging the opticnerve which, in turn, can cause blind spots in the field of vision.Total blindness may occur if the entire optic nerve is damaged.

One known manner of measuring intraocular pressure is to use an externalpressure measurement device that acquires intraocular pressure readingsfrom outside of the eye. One known pressure measurement device is knownas a tonometer, which measures an external deformation of an eye andrelates that measurement to intraocular pressure. Such externalmeasurement devices, however, may not have the desired level of accuracysince they operate in an external environment rather than within the eyeitself. Further, such devices do not provide for continuous monitoringof intraocular pressure since a tonometer must be utilized each timeintraocular pressure is to be determined and, therefore, providesdiscontinuous intraocular pressure monitoring.

It is also known to implant a sensor into an eye for purposes ofmeasuring an electrical parameter related to intraocular pressure, andto use telemetry to obtain an electrical parameter measurement andrelate the electrical parameter measurement to intraocular pressure. Inone known system, an external instrument generates a signal to remotelyenergize an in vivo intraocular pressure sensor. The response generatedby the in vivo sensor is measured and correlated to intraocularpressure.

For example, referring to FIG. 1, a known intraocular telemetry system10 includes an external system 20 and an internal or implantedintraocular sensing circuit 30. The external system 20 includes anexcitation circuit 21 and a measurement device 22. The sensing circuit30 typically includes a resistor (R_(sensor)) 33 and an inductor(L_(sensor)) 34 and a capacitor (C_(sensor)) 35. The capacitor 35 may beconfigured to vary with the intraocular pressure applied to thecapacitor 35.

The excitation circuit 21 typically includes an inductor (L) 24. Duringuse, the excitation circuit 20 generates energy, which is delivered tothe sensing circuit 30 by inductive coupling between the inductors 24,34, thereby energizing the sensing circuit 30. The resulting response(e.g., resonant frequency or impedance) of the sensing circuit 30 ismeasured by the measurement device 22 and correlated to intraocularpressure.

The implanted sensing circuit 30 is essentially an RLC resonancecircuit. The resonant frequency and the Quality (Q) factor of thecircuit 30 are determined by resistance, capacitance and inductanceparameters as provided by Resonant Frequency (f)=1/(2π√(LC)); and QFactor=1/R (√(L/C)). A change of capacitance causes a shift in resonantfrequency of the implanted sensor circuit 30, which can be wirelesslymeasured by the external measurement device 22. Examples of suchintraocular implants and telemetry systems are described in U.S. Pat.No. 6,579,235 to Abita et al., “Passive Silicon Transensor Intended forBiomedical, Remote Pressure Monitoring,” by Backlund et al., “A systemfor wireless intra-ocular pressure measurements using a siliconmicromachined sensor,” by Rosengren et al., and “A system for passiveimplantable pressure sensors”; by Rosengren et al.

One known capacitor for use in intraocular pressure sensors ismanufactured using MEMS technologies and includes a membrane, a flatbottom portion and a chamber. The capacitor is part of a pressure sensorthat is implantable to monitor pressures through a remote telemetryconnection. Another known capacitor device used in pressure sensors isreferred to as a comb-drive capacitor unit. One known capacitor unit isdescribed in “Design and Simulation of a MEMS-Based Comb-Drive PressureSensor for Pediatric Post-Operative Monitoring Applications,” byDuck-Bong Seo et al. Seo et al. describe an implantable MEMS-basedpressure sensor to monitor pressures through a remote telemetryconnection in the context of monitoring pressures of the right side ofthe heart following surgery. Seo et al. show a flat membrane and a combdrive and explain that a change of overlapping area changes thecapacitance of the device, and that no bending or other deformation ofthe membrane was found for the comb-drive sensor.

While known sensor devices and telemetry systems may provide someimprovements over known external pressure measurement devices, they canbe improved. For example, certain known sensor devices presentperformance, biocompatibility, packaging and/or size challenges. Certainknown devices also lack sensitivities and detection ranges suitable forvarious biomedical applications. Further, certain known devices utilizewafer bonding techniques, which typically require additional fabricationtime and result in larger or thicker devices. Additionally, bondingoften results in reduced yield rate, e.g. due precise componentalignment requirements. Thus, devices that are fabricated using waferbonding are not desirable. Certain known devices also may not beadaptable to commercial fabrication on a large scale. Additionally, theinductor element of the implanted sensor circuit can be improved toprovide a more effective sensor circuit and more accurate intraocularpressure determinations. Known devices may also require larger incisionsor blades for implantation of sensor devices due to their large size.Such incisions are not desirable. Further, certain known implantsrequire sutures to remain implanted in the eye, which are also notdesirable.

Therefore, it would be desirable to have implantable sensor devices thatcan be fabricated using known micromachining and MEMS technologies. Itwould also be desirable to have implantable sensor devices that aresufficiently small or miniature in size so that they may be deliveredthrough a needle rather than through a large incision using a blade. Itwould also be desirable to have sensor devices that may be implantedwithout the need for sutures and in various locations of an eye.Further, it would also be desirable to have biocompatible andimplantable microfabricated sensor devices with improved capacitor andinductor components for enhanced sensitivity, dynamic range andaccuracy. It would also be desirable to continuously and passivelymonitor intraocular pressure by telemetry using such sensor devices.Such capabilities would enhance biomedical applications andpressure-dependent physical conditions and diseases including monitoringof intraocular pressure.

SUMMARY

According to one embodiment, a variable capacitor of a microfabricatedimplantable pressure sensor includes a substrate, a flexible memberhaving edges disposed on the substrate and a plurality of capacitorelements. The substrate defines a plurality of channels. The flexiblemember includes a middle portion that is raised above the substrate,thereby defining a chamber between the substrate and the middle portion.The capacitor elements extend indirectly from the flexible member. Thecapacitor is configured so that changes of fluid pressure causes theflexible member, e.g., the middle portion, to deform, thereby causingcapacitor elements to move within respective channels. As a result,capacitance varies due to changes in an overlapping area of thecapacitor elements and the substrate.

According to another embodiment, a microfabricated implantable pressuresensor includes a variable capacitor and an inductor. The variablecapacitor and the inductor are electrically connected to each other. Thevariable capacitor includes a substrate, a flexible member and aplurality of capacitor elements. The substrate defines a plurality ofchannels, and edges of the flexible member are on the substrate. Amiddle portion of the flexible member is raised above the substrate,thereby defining a chamber. Capacitor elements extend indirectly fromthe flexible member. Fluid pressure changes on the middle portion causethe middle portion to move, thereby causing the capacitor elements tomove within respective channels and causing capacitance to vary withchanges in an overlapping area of the capacitor elements and thesubstrate. An electrical circuit including the variable capacitor andthe inductor can generate a detectable resonant frequency shift inresponse to a change of fluid pressure on an outer surface of theflexible member.

Another embodiment is directed to a microfabricated implantable pressuresensor that includes a substrate, a flexible member disposed on thesubstrate, a variable capacitor and a variable inductor. A chamber isdefined between the substrate and the flexible member, and the variablecapacitor and variable inductor are electrically connected to eachother. The flexible member carries components of the variable capacitorand also carries components of the variable inductor. With thisconfiguration, the flexible member can be moved in response to fluidpressure changes on an outer surface of the flexible member to varycapacitance and inductance.

A further alternative embodiment is directed to a method of implantingan intraocular pressure sensor at a treatment site in an eye of apatient. The method includes inserting a needle into the eye, deliveringan intraocular pressure sensor having a variable capacitor and aninductor through the needle and implanting the intraocular pressuresensor deployed from the needle at the treatment site in the eye.

Another embodiment is directed to a method of measuring intraocularpressure. The method includes generating a signal with an externalinstrument and energizing a pressure sensor implanted in an eye by thegenerated signal. The pressure sensor includes an inductor and avariable capacitor that includes a flexible member and capacitorelements extending indirectly from the flexible member and movablewithin channels defined within a substrate. The method also includesmeasuring an interaction between the signal generated by the externalinstrument and the pressure sensor and determining intraocular pressurebased on the measured interaction.

In one or more embodiments, capacitor elements extend indirectly from aflexible member by an indirect connection, e.g., by an indirectconnector including an intermediate member and a cross bar or member.The capacitor elements are carried by the cross bar or member, which isconnected to or extends from an intermediate member, which extendsbetween the flexible member and the cross bar or member. Thus, capacitorelements that move within channels do not extend directly from theflexible member. In one or more embodiments, the middle portion of theflexible member may be deformed in a non-linear manner, e.g., to assumea bowl-like shape, while the intermediate member/cross bar configurationpermits the capacitor elements to remain movable within respectivechannels in a direction that is perpendicular to a plane defined by atop surface of the substrate. The intermediate member and at least onecapacitor element may lie within a common vertical plane, and at leastone capacitor element may lie within a vertical plane that is offsetfrom and parallel to a vertical plane defined by the intermediatemember.

In one or more embodiments, a middle portion of a flexible member may beflexible and resilient (e.g., made of Parylene) so that movement ordeformation of the flexible member alters the overlapping area ofcapacitor elements and the substrate, thereby changing capacitance.Channels in the substrate and capacitor elements may form mating combstructures.

In one or more embodiments, variable capacitor and the inductorcomponents are configured to detect fluid pressure changes with asensitivity of about 1 mmHg within a fluid pressure range of about 1-50mmHg.

In one or more embodiments, the inductor may be stationary and have afixed inductance and be formed by a stack of insulated inductor elementsthat encircle a variable capacitor. Inductor components may extendthrough the entire substrate or extend partially through or be depositedon the substrate. The inductor may also be in the form of a ring, whichcan be collapsed or compressed configuration for delivery through aneedle, e.g., a 20-25 gauge needle, and expanded when delivered at thetreatment site. Embodiments also provide for suture-less implantation.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout and in which:

FIG. 1 illustrates a known telemetry system for monitoring intraocularpressure;

FIG. 2 is a perspective cross-sectional view of a variable capacitor ofa microfabricated implantable pressure sensor constructed in accordancewith one embodiment;

FIG. 3 is a cross-sectional view of the variable capacitor shown in FIG.2 in which a flexible member is in an initial or relaxed state whenexternal fluid pressure is less than an internal chamber pressure;

FIG. 4 is a cross-sectional view of a variable capacitor shown in FIG. 2in which a flexible member is in a compressed or deformed state whenexternal fluid pressure is greater than the internal chamber pressure;

FIG. 5 further illustrates a channel formed within a substrate and acapacitor element extending from a flexible member and being moveablewithin the channel to alter the overlapping area and capacitance;

FIG. 6 is a graph showing a relationship between the overlapping areaand change in capacitance achieved with the capacitor configurationshown in FIGS. 2-5;

FIG. 7 is a partial cross-sectional view of a substrate and a capacitiveelement at a first depth within a channel, corresponding to overlappingarea A1 in FIG. 6;

FIG. 8 is a partial cross-sectional view of a substrate and a capacitiveelement at a second depth deeper within a channel, corresponding tooverlapping area A2 in FIG. 6;

FIG. 9 is a partial cross-sectional view of a substrate and a capacitiveelement at a third depth deeper within a channel, corresponding tooverlapping area A3 in FIG. 6;

FIG. 10 is a graph showing a relationship between changes of capacitanceand pressure, and measurement sensitivity achieved with the capacitorconfiguration shown in FIGS. 2-5;

FIG. 11 is a perspective view of a lump inductor of an implantablepressure sensor having integrated metal lines according to oneembodiment;

FIG. 12 is a perspective cross-sectional view of a lump inductorconstructed having stacked metallic layers separated by insulativematerial according to another embodiment;

FIG. 13 is a perspective cross-sectional view of a lump inductorconstructed having metallic elements formed through or embedded within asubstrate according to another embodiment;

FIG. 14 is a perspective view of a lump inductor having metallicelements formed through or embedded within a substrate and a foldable orrollable inductor sheet or ring according to a further embodiment;

FIG. 15 further illustrates a structure of the foldable or rollableinductor sheet or ring of the inductor shown in FIG. 14 in accordancewith one embodiment;

FIG. 16A is a flow diagram illustrating one embodiment of a method offabricating an implantable pressure sensor having a variable capacitorand a lump inductor in which inductor elements are formed by metal linesextending through a substrate;

FIG. 16B illustrates an alternative sensor configuration having avariable capacitor and a variable inductor that can be fabricated usingprocess steps shown in FIG. 16A;

FIG. 17 is a table summarizing expected electrical parameters ofmicrofabricated pressure sensors constructed according to embodimentsand having a variable capacitor shown in FIGS. 2-4 and different lumpinductor configurations shown in FIGS. 12-15;

FIG. 18 is a perspective cross-sectional view of a microfabricatedimplantable pressure sensor including a variable capacitor, a lumpinductor and a solid substrate according to another embodiment;

FIG. 19 is a cross-sectional view of another configuration of thepressure sensor shown in FIG. 18 having a lump inductor includingmultiple metallic layers;

FIG. 20 is a cross-sectional view of a microfabricated implantablepressure sensor having a variable capacitor and a lump inductorconstructed according to another embodiment;

FIG. 21 is a flow diagram illustrating one embodiment of a method offabricating an implantable pressure sensor constructed as shown in FIG.20;

FIG. 22 is a flow diagram illustrating another embodiment of a method offabricating an implantable pressure sensor having a recessed cavity on abackside of a substrate;

FIG. 23 is a perspective cross-sectional view of a microfabricatedimplantable pressure sensor having a variable capacitor and a variableinductor according to a further embodiment;

FIG. 24 is a cross-sectional view of a microfabricated implantablepressure sensor having a variable capacitor and a variable inductoraccording to another embodiment; and

FIG. 25 is a flow diagram illustrating one embodiment of a method offabricating an implantable pressure sensor having a variable capacitor,a variable inductor and a recessed cavity.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS

Certain embodiments are directed to a variable capacitor that is acomponent of a microfabricated implantable pressure sensor for use invarious biomedical applications. A variable capacitor constructedaccording to embodiments includes a substrate having trenches orchannels defined therein and a flexible member. A portion of theflexible member is raised above the substrate. Capacitor elements extendindirectly from the flexible member and movable together andsimultaneously within the channels, thereby varying capacitance as aresult of changing the overlapping area of the substrate and capacitorelements. Certain other embodiments are directed to a microfabricatedimplantable pressure sensor and configurations of a variable capacitorand an inductor. An inductor may have a fixed or variable inductance.The inductor may be fixed or stationary, or be movable, e.g., with acomponent of the variable capacitor. Certain other embodiments aredirected to a microfabricated sensor having a variable capacitor and avariable inductor that are carried by or embedded within a commonflexible member. Certain embodiments are directed to methods offabricating implantable pressure sensors using surface micromachiningand MEMS technologies.

Embodiments advantageously provide implantable pressure sensors that maybe fabricated using known micromachining and MEMS technologies and areof a miniature size so that they may be delivered through a needle andimplanted in various locations without the need for sutures. Embodimentsalso advantageously provide biocompatible pressure sensors havingvariable capacitors, lump/variable inductors with enhanced accuracy,sensitivity and range for use in various biomedical applicationsincluding passive monitoring of intraocular pressure using telemetry andother biomedical applications involving, e.g., aneurysms and the brain.

FIGS. 2-10 illustrate embodiments of a variable capacitor of amicrofabricated implantable pressure sensor for use in biomedicalapplications. The variable capacitor includes a flexible member, aportion of which is raised above a substrate and capacitor elements orplates that are moveable within channels or trenches formed within thesubstrate to vary capacitance. FIGS. 11-17 illustrate different lump orfixed inductor configurations that may be used with a variable capacitorand electrical characteristics thereof and related methods offabrication. FIGS. 18-21 illustrate embodiments of a microfabricatedimplantable pressure sensor for use in biomedical applications andhaving a flexible member, a portion of which is raised above a substratethat does not include channels or trenches, variable capacitance andfixed inductance. FIGS. 23-25 illustrate embodiments of amicrofabricated implantable pressure sensor for use in biomedicalapplications and having a flexible member that carries elements of avariable capacitor and also elements of a variable inductor.

Referring to FIGS. 2-4, a variable capacitor 200 constructed inaccordance with one embodiment and configured for use in amicrofabricated implantable pressure sensor includes a substrate 210, aflexible member 220 disposed on the substrate 210, and a capacitorcomponent 230 that includes a plurality of capacitor elements 232extending indirectly from the flexible member 220. The capacitorelements 232 are movable within trenches, grooves or channels (generallychannels 216) defined through the substrate 230, e.g., partially throughthe substrate 230 as shown in FIGS. 2-4. In the illustrated embodiment,all of the capacitor elements 232 are the same length, but othercapacitor element 232 configurations may be utilized.

Movement of the capacitor elements 232 to different depths within thechannels 216 alters the overlapping area of the capacitor elements 232and the substrate 220. Changing the overlapping area alters capacitanceand the resonant frequency response of a sensor circuit that includesthe variable capacitor 200. For example, FIG. 3 illustrates a pluralityof capacitor elements or plates 232 positioned at a first depthresulting in an overlapping area A1, whereas the overlapping areaincreases by ΔA to A2 when the capacitor elements 232 are moved deeperdown into the channels 216.

The substrate 210 may be composed of silicon and may be in the form of awafer having a thickness of about 500 microns. Although thisspecification refers to silicon, the substrate 210 may be composed ofother materials including a conductive polymer or another suitablemicromachinable substrate material having sufficiently highconductivity. The substrate 210 has a top surface 212 and a bottomsurface 214. One or more channels 216 are formed through the top surface212 of the substrate, thereby forming corresponding projections, wallsor fingers 218. In the illustrated embodiment, the channels 216 andprojections 218 form a comb structure.

In the illustrated embodiment, the substrate 210 defines a plurality ofchannels 216, e.g., five channels 216, and four correspondingprojections 218. It should be understood, however, that the substrate210 may define other numbers of channels 216, e.g., about 3 to 10channels 216. The number of channels may depend on the capacitor 200configuration, e.g., the width of the substrate 210 and/or the number ofcapacitor elements 232. Further, although the illustrated embodimentshows channels 216 and projections 218 that are the same width, thechannels 216 and projections 218 may have different widths to providedifferent variations of capacitance and to accommodate different numbersof channels 216 and different capacitor element 232 configurations.

For example, in embodiments including a 500 micron substrate 210, eachchannel 216 may have a depth of about 200 microns, a width of about 20microns, a spacing (projection 218 width) of about 20 microns. Thecapacitor elements 232 may be movable by about 50 microns within thechannels 216, resulting in an overlapping area of the capacitor elements232 and substrate 210 that may range from about 10⁶ to about 10⁷ squaremicrons. It should be understood that other dimensions andconfigurations may be utilized as necessary.

The flexible member 220 includes an outer or top; surface 221 and aninner surface 222. First and second edges or bottom surfaces 223, 224are disposed on, connected to, formed on, or sealed to the top surface212 of the substrate 210. During fabrication of the variable capacitor200, another material or coating, such as a layer of silicon dioxide(not shown in FIGS. 2-4), may be applied on the top surface 212 of thesubstrate 210. Thus, the edges 223, 224 of the flexible member 220 maybe in direct contact with a silicon dioxide layer rather than the topsurface 212. For ease of explanation and illustration, FIGS. 2-4 showedges 223, 224 being disposed on, connected to or formed on the topsurface 212 of the substrate 210, whether such contact is direct orindirect as a result of an intermediate silicon dioxide layer.

The flexible member 220 also includes a middle portion 225 that extendsbetween the first and second edges 223, 224. The middle portion 225 israised above the top surface 212 of the substrate 210, thereby definingan inner space or chamber 226 between the top surface 212 and the innersurface 222 of the flexible member. The capacitor 200 is eventuallysealed so that the inner space or chamber 226 is also sealed and has afixed internal or chamber pressure (P_(c)).

In the illustrated embodiment, the middle portion 225 includes first andsecond arcuate or “shoulder” sections 227, 228. In the illustratedembodiment, each shoulder section 227, 228 extends inwardly and upwardlyfrom respective first and second edges 223, 224 to a middle section 229that extends between the shoulder sections 227, 228. In the illustratedembodiment, the middle section 229 is flat and parallel to the topsurface 212 of the substrate 210, whereas the shoulder sections 227, 228extend upwardly in some manner (e.g., as a result of having an arcuateshape) so that the middle section 229 is raised above the substrate 210.It should be understood that the middle portion 225 may have othershapes and that the shoulder sections 227, 228 may be arcuate orshoulder shapes or other shapes as necessary in order to raise themiddle section 229 above the substrate 210.

The flexible member 220 is made of a material that allows the middleportion 225, e.g., the middle section 229 and/or one or more shouldersections 227, 228 depending on the capacitor 200 configuration and fluidpressure application, resulting in deformation, deflection or bending ofthe middle portion 225 under fluid pressure (P_(f)) if the fluidpressure is greater than the internal chamber pressure (Pc) (as shown inFIG. 4). The flexible member 220 may be resilient to return from adeformed shape (as shown in FIG. 4) to an initial or relaxed shape (asshown in FIG. 3) when the external fluid pressure is less than theinternal chamber pressure.

For this purpose, the flexible member 220 may be composed of a materialhaving a suitable Young's modulus of about 1 GPa to about 10 GPa, e.g.,about 4 GPa. One example of a suitable material for the flexible member120 is Parylene, e.g., Parylene C, D, N, F, HT, A and AM. For ease ofexplanation, reference it made to the flexible member 220 being made ofa polymer or Parylene, but it should be understood that the flexiblemember 120 may be composed of other suitable materials that providedesired flexibility and/or resiliency attributes. Selection of flexiblemember 120 materials may also depend on, for example, ease ofmicromachining, CMOS/MEMS process compatibility and biocompatibility(e.g., USP Class VI implantable grade).

In one embodiment, the flexible member 220 may be made of Parylene, havea width of about 500 microns, and the shoulder sections 227, 228 may beconfigured so that the middle section 229 is raised above the topsurface 212 of the substrate 210 by about 10 microns. The middle portion225 may be moved or deflected by about 10 microns towards the substrate210. It should be understood that these dimensions are provided as oneexample of how a variable capacitor 200 having a raised flexible member220 may be implemented, and other configurations may be utilized fordifferent applications.

Referring to FIGS. 2-4, and with further reference to FIG. 5, capacitorelements 232 may be in the form of fingers or plates that extendindirectly from the flexible member 220 and are arranged in a combstructure. In the illustrated embodiment, capacitor elements 232 extenddirectly from, or are carried by, one or more cross bars or members 234.An intermediate member 236 extends between the flexible member 220 andthe cross bars or members 234. FIGS. 2-4 illustrate an embodiment thatincludes a single intermediate member 236 that connects the middlesection 229 of the flexible member 220 and the cross-bar member 234.Portions of the chamber 226 are defined by the inner surface 222 of themiddle section 229, cross bar members 234, the intermediate member 236extending between the flexible member and a cross bar member 234.

According to one embodiment, the number of intermediate members 236 isless than the number of capacitor elements 232. In the illustratedembodiment, a single intermediate member 236 joins the middle section229 and a cross bar 234 that carries a plurality of capacitor elements232. This configuration advantageously provides a flexible member 220having sufficient flexibility and advantageously provides linear orvertical movement or substantially linear or vertical movement ofcapacitor elements 232 within channels 216 even when the flexible member220 is deformed.

More specifically deformation of the flexible member 220 by fluidpressure results in downward movement of the flexible member anddownward movement of the intermediate member 236 extending from theflexible member. This results in downward movement of the capacitorelements 232 carried by the cross bar 234, which extends from theintermediate member 236. The structural configuration of embodimentsadvantageously prevents outward bowing of capacitor elements 232 thatmay result if the capacitor elements 232 extended directly from theflexible member 220 (i.e., without any intermediate member 236, as inknown comb structure devices), thereby causing capacitor elements 232 toscrape against inner surfaces of the channels 216, or causing thecapacitor elements 232 to not be positioned within the channels 216depending on the configuration of the capacitor. Thus, embodimentsadvantageously utilize an intermediate member 236/cross bar 234configuration so that capacitor elements 232 extend indirectly from theflexible member, thereby preventing the capacitor elements 232 frombeing pushed out at an angle when the flexible member 220 is deformed,e.g., in a bowl-like shape, by fluid pressure.

It should be understood that other structural configurations may beutilized while achieving these advantages. For example, rather thanhaving a single intermediate member 236, other numbers of intermediatemembers 236 may be utilized so long as the number of intermediatemembers 236 provides sufficient flexibility and maintains the verticalorientation of the capacitor elements 232 when the flexible member 220is deformed.

In the illustrated embodiment, at least one capacitor element 232 isin-line with, or within the same vertical plane defined by, theintermediate member 236, and at least one other capacitor element 232 iswithin a vertical plane that is offset from the vertical plane definedby the intermediate member 236. In the illustrated example, the middlecapacitor element 232 lies within the same vertical plane defined by theintermediate member 236, and the other capacitor elements lie withindifferent vertical planes and are parallel to the plane defined by theintermediate member 236 and the middle capacitor element 232. In otherembodiments, the capacitor elements 232 may be arranged so that nocapacitor element 232 is in-line with or within the same vertical planedefined by the intermediate member 236, but all capacitor elements 232are parallel to the plane defined by the intermediate member 236. Theparticular configuration utilized may depend on, e.g., the number ofintermediate members 236, the number of capacitor elements 232 and thearrangement of these components.

Referring to FIG. 5, capacitor elements 232 are configured and have asuitable shape and size so that they may move with the flexible member220 within channels 216, e.g., within channels 216 of a correspondingsubstrate 120 comb structure. According to one embodiment, a capacitorelement 232 is a conductive material 510, such as a metal, and may beoptionally coated with an insulation material 512. In anotherembodiment, the capacitor element 232 may include a metal coating thatis applied over a conductive, non-metallic material. A channel 216 mayalso include an insulative coating 520 and a conductive or metal coating522 that is applied within the channel 216 using metallization.

During use, the flexible member 220 having capacitor elements 232extending there from is used as a variable capacitor electrode, and thesubstrate 110 is used as a ground electrode. If the internal chamber 226pressure is greater than the external fluid pressure, then the flexiblemember 220 will not be deformed or bent and will retain its original orinitial shape. If the fluid pressure exceeds the chamber 226 pressure,then the middle portion 229, e.g., the middle section 225 of theflexible member 220, will be deformed or deflected by the fluidpressure. The flexible member 220 may be sufficiently thin (e.g., about10 microns) so that the amount of deflection of the middle portion 229is proportional to the difference between the external fluid pressureand the internal chamber pressure, ((δ)(α)(ΔP)). At the same time, theposition of the capacitor elements 232 extending from the flexiblemember 220 is changed, i.e., the capacitor elements 232 move with themoving flexible member 220.

As a result, the effective overlapping area between interdigitatedelectrodes is changed which, in turn, alters the capacitance across theelectrodes. More specifically, the capacitance increases as thecapacitor elements 232 are moved deeper within respective channels 216and the overlapping area of the substrate 220 and the capacitor elements232 increases, and capacitance decreases as the capacitor elements 232are moved to a shallower depth within the channel 216 and theoverlapping area of the substrate 220 and the capacitor elements 232decreases.

For example, referring to FIG. 6, capacitor elements 232 may assume aninitial, relaxed position, generally illustrated as (0,0). The initialposition may be the capacitor elements 232 being positioned partiallywithin respective channels 216. Alternatively, a capacitor element 232may be positioned outside above the channels 216, e.g., above the topsurface 112 in the illustrated example. The initial relaxed position maydepend on the variable capacitor 200 configuration, e.g., how far theflexible member 220 may be deflected or deformed and the length of thecapacitor elements 232.

FIG. 7 illustrates one example in which the initial, relaxed position isa position in which distal portions of capacitor elements 232 arepositioned partially inside respective channels 216. When the chamber226 pressure is greater than the external fluid pressure, the flexiblemember 220 is in its initial, relaxed state, and the capacitor elements232 are positioned at a first depth within the channels 216. Thisarrangement results in an initial overlapping area (A1) of the distalportions of the capacitor elements 232 and the substrate 210, and acorresponding capacitance C1.

Referring to FIGS. 6 and 8, as fluid pressure on the outer surface 221of the flexible member 220 increases, the fluid pressure will exceed theinternal chamber 226 pressure, causing the flexible member 220 to bendor deflect towards the substrate 210. This causes the capacitor elements232 to be moved from the initial depth to a second, deeper depth withinthe channels 216. This movement of the flexible member 220 results inthe overlapping area of the capacitor elements 232 and the substrate 210to increase from A1 to A2 and results in a corresponding increase incapacitance from C1 to C2.

Similarly, as shown in FIGS. 6 and 9, as fluid pressure increasesfurther, the flexible member 220 will bend or deflect towards thesubstrate 210 to a greater degree, thereby moving the capacitor elements232 to a third, depth within the channels 216. This movement results inthe overlapping area of the capacitor elements 232 and the substrate 210to increase from A2 to A3 and a corresponding increase in capacitancefrom C2 to C3.

The capacitance behavior of this structure can be expressed as ΔC=(εA/d)(α)(δ)(α)(ΔP), where ΔC=change of capacitance for a deflection of theflexible member 220 and corresponding movement of capacitive elements232 within channels 216; ε=permittivity of the channel 216 space;A=overlapping area of capacitor elements 232 and substrate 210;d=distance between a conductive portions 510 of a capacitor element 232and a conductive layer 520 of the channel 216 of the substrate 210; α isthe proportional symbol and ΔP=the change in fluid pressure on theflexible member 220.

FIG. 10 illustrates how a change in capacitance may be correlated to achange in fluid pressure on the flexible member 220. In the illustratedexample, a 0.4 pF change of capacitance corresponds to a pressure changeof 1 mm Hg. Thus, embodiments are capable of pressure measurements with1 mm Hg sensitivity.

The total capacitance may be expressed as C(total)=C0+ΔC(ΔP) whereC(total)=total capacitance; C0=a fixed capacitance (when ΔP=0);ΔC=change of capacitance as a function of pressure difference ΔP on theflexible member 220 and ΔP=pressure difference on the flexible member220. The total capacitance should be sufficiently high to allow avariable capacitor 200 to be used in telemetry systems (e.g., in thesystem generally illustrated in FIG. 1). Total capacitance may beincreased by increasing the area of the capacitor elements 232 (largerelectrode overlapping area), providing a larger number of capacitorelements 232, structuring the flexible member 220 so that it may bedeflected to greater depths within channels 216 to increase overlappingareas, and decreasing the distance between interdigitated electrodes.

Additional considerations for effective telemetry include having apressure sensor with sufficiently high inductance and sufficiently highcoupled capacitance in order to allow the resulting resonant frequencyof the sensor circuit to lie within a reasonable detection range. Forexample, the resonant frequency of an implantable sensor circuit shouldlie between 10-500 MHz for telemetry involving biomedical applications.For this purpose, in addition to having a variable capacitor 200 andsufficient capacitance as discussed above with reference to FIGS. 1-10,microfabricated implantable pressure sensors should also have inductorelements that allow the sensor to be implantable and provide electricalcharacteristics (e.g. resonant frequency) suitable for use in biomedicalapplications and telemetry. FIGS. 11-15 illustrate different embodimentsof pressure sensors having lump inductors or inductors having a fixedinductance.

FIG. 11 illustrates a lump inductor 1110 constructed in accordance withone embodiment for use in a microfabricated implantable pressure sensor1100 includes a variable capacitor 200 (not illustrated in FIG. 11 forclarity). Further, FIG. 12 is a perspective cross-sectional viewillustrating metallic layers 1211 along two sides of the variablecapacitor 200 in order to illustrate how the variable capacitor 200 andthe inductor 1210 may be integrated within the sensor 1200, but itshould be understood that the stacked metallic layers 1211 are arrangedaround the variable capacitor 200.

The inductor 1110 is formed by metal lines 1112 that are integratedwithin the top surface 212 of the substrate 210 and surround thevariable capacitor 200. In the illustrated embodiment, a single wire1112 is wound in a spiral pattern around the variable capacitor 200. Oneexample implementation of the inductor 1110 shown in FIG. 11 may includea metallic line or element 1112 having a thickness of about 2 microns, awidth of about 20 microns, and being wound to form about fiveoverlapping sections. Overlapping metal lines 1112 may be spaced apartby about 10 microns.

Referring to FIG. 12, in another embodiment, a microfabricated pressuresensor 1200 includes an inductor 1210 that is formed as a stack ofmetallic layers 1211 that are fabricated using surface-micromachiningmethods. In this embodiment, the inductor 1210 is arranged so thatalternating insulative layers 1212 and metallic layers 1211 are stackedtogether. This inductor configuration may be particularly suited forconfigurations that required increased lump inductance and lumpcapacitance. The insulative layer 1212 may be a polymer such as Paryleneor the same material that is used to form the flexible member 220. Allof the metallic layers 1211 may be embedded within an insulativematerial 1212, or a top metallic layer 1211 may be exposed (as shown inFIG. 12). In one embodiment, the inductor 1210 may include a stack ofabout two to four metallic layers 1211. The thickness of a metalliclayer 1211 may be about 2 microns, a width of a metallic layer 1211 maybe about 20 microns and the thickness of the insulative layer 1212between metallic layers 1211 may be about 2 microns.

Referring to FIG. 13, it may be desirable to increase inductance whilereducing resistance in order to increase the quality (Q) factor forhigher sensing capabilities in terms of both sensitivity and sensingdistance. For this purpose, a microfabricated pressure sensor 1300 mayinclude a variable capacitor 200 (as shown in FIGS. 1-10) and a highaspect ratio inductor 1310. FIG. 13 is a perspective cross-sectionalview illustrating the inductor 1210 elements along two sides of thevariable capacitor 200 in order to illustrate how the variable capacitor200 and the inductor 1310 may be integrated within the sensor 1300, butit should be understood that the metal lines 1311 are arranged aroundthe variable capacitor 200.

The inductor may include thick metal lines 1311 that fill channels 216that are formed completely through the portions of the substrate 210. Inother embodiments, the metal lines 1311 may fill channels 216 formedpartially through the substrate 210 depending on the desired inductanceand resistance. The high aspect ratio inductor 1310 configuration shownin FIG. 13 is well suited to maximize the capacitance and inductance ofthe sensor 1300 while reducing resistance as a result of the dimensionsof the thick metal lines 1311 based on the expression R=ρL/A, whereρ=resistivity of the metal material, L=length of the metal line, andA=area of the metal line. For example, the thickness of the substrate210 may be about 500 microns, metal lines 1311 may extend through thesubstrate 210 to have a depth that is also about 500 microns, the widthof the metal lines 1311 may be about 20 microns and the metal lines 1311may extend along the length of the substrate 210, e.g., about 3millimeters.

Referring to FIGS. 14 and 15, in another embodiment, a microfabricatedpressure sensor 1400 may include a variable capacitor 200 and a lumpinductor 1410 in the form of an inductor sheet. For purposes ofillustration, not limitation, the sensor 1400 is shown as having aninductor sheet 1410 that is coupled to metal lines 1311 of the highaspect ratio inductor 1310 shown in FIG. 13. In other embodiments, theinductor sheet 1410 may be used as the sole inductor element, or incombination with other types of inductors, e.g., as shown in FIGS. 11and 12. Thus, FIGS. 14 and 15 are provided as one example of howembodiments may be implemented.

In the illustrated embodiment, the inductor sheet 1410 has a circularshape (when in an expanded or relaxed shape) and includes alternatingmetallic layers 1411 and insulative layers 1412. The metallic layers1411 may be platinum, titanium and gold, or another suitablebiocompatible metal or conductive materials. The insulative layers 1412may be a polymer such as Parylene.

The inductor sheet 1410 is preferably configured for implantationthrough a clinical gauge needle (e.g., having a 20-25 gauge size). Forthis purpose, the inductor sheet 1410 may be configured to assume astressed or compressed shape when being delivered through a needle andan expanded or relaxed shape after the sensor 1400 is deployed from theneedle and implanted. For example, the inductor sheet 1410 may be rolledor folded while positioned within the needle and may expand to assume acircular shape (as shown in FIGS. 14 and 15) when the pressure sensor1400 is deployed from the needle.

FIG. 16A illustrates an embodiment of a method 1600 of fabricating amicromachined pressure sensor having a variable capacitor (e.g., asshown in FIGS. 1-10) and a lump inductor (e.g., the inductor 1310 asshown in FIG. 13). It should be understood that method steps shown inFIG. 16A can be utilized and/or adapted to fabricate pressure sensorshaving other variable capacitors and other lump inductors (e.g., asshown in FIGS. 11, 12, 14 and 15). For ease of explanation, reference ismade to a method for fabricating the pressure sensor having a variablecapacitor and lump inductor shown in FIG. 13.

At stage 1605, a substrate 210, such as a silicon wafer, is provided.The substrate 210 may have a thickness of about 500 microns. Thesubstrate 210 is etched, e.g., deep reactive-ion etching (DRIE). In theillustrated embodiment, DRIE may be used to etch partially through acentral portion of the substrate 210 to form channels (for the eventualvariable capacitor 200) and to form other channels 216 completelythrough the substrate 210 (for the eventual inductor 1310). The width ofthe channels 216 in the central portion of the substrate 210 may beabout 20 microns, and the depth of the channels 216 in the centralportion of the substrate 210 may be about 200 microns. The width of theother channels 216 formed through the substrate 210 may also be about 20microns. A tissue anchor (not shown in FIG. 16A) may be created on thebackside 214 of the substrate 210. One example of a suitable tissueanchor is described in U.S. Publication No. 2006/0247664, entitled“Micromachined Tissue Anchors for Securing Implants Without Sutures byE. Meng et al., the contents of which are incorporated herein byreference.

At stage 1610, a first insulative layer 520 (e.g., as shown in FIG. 5)is deposited over the top surface 212 of the substrate 210. Theinsulative layer 520 may be Parylene and may have a thickness of about 2microns. As shown in FIG. 16A, the first Parylene layer 520 is appliedand patterned to coat surfaces that were exposed as a result of theetching at stage 1605, i.e., the inner surfaces of the open channels 216formed partially and completely through the substrate 210

At stage 1615, metal electroplating is performed on the open channels216 that were formed through the substrate 210 so that these channels216 are filled with metal 1311 (as further illustrated in FIG. 13).These metal-filled channels or lines 1311 will eventually form the highaspect ratio inductor 1310 that is integrated within the substrate 210.

At stage 1620, surface metallization is performed on channels 216 thatwere formed partially through the substrate 210, thereby forming a layerof metal 522 over the first Parylene layer 520 (as further illustratedin FIG. 5)

At stage 1625, a first sacrificial coating of photoresist 1626 isapplied (e.g., by spin coating) over a portion of the substrate 210. Thethickness of the first photoresist coating 1626 may be about 10 microns.One suitable photoresist 1626 that may be utilized with embodiments is alayer of AZ4620 type photoresist (supplied by Clariant Corp., Charlotte,N.C.). The photoresist 1626 may be hard-baked at about 120° C. forsmoothing of edges and degassing purposes. In the illustratedembodiment, the first photoresist coating 1626 is applied over themetal-filled channels 1311 positioned between other open channels 216formed partially through the substrate 210.

At stage 1630, Parylene is applied and patterned a second time to fillwith Parylene open channels 216 that were previously coated with metal,and to coat the photoresist 1626 with Parylene. The second Parylenelayer may have a thickness of about 2 microns and will eventually formcapacitor elements 232 and the cross bar 234 (as further illustrated inFIGS. 2 and 13).

At stage 1635, a second sacrificial photoresist coating 1636 is appliedand patterned over the second Parylene coating that forms capacitorelements 232 and cross bar 234 elements, over portions of the substrate210 and over channels 216 filled with metal 1311. The thickness of thesecond photoresist coating 1636 may be about 15 microns.

At stage 1640, metal connections are formed on electrodes (not shown forclarity) for purposes of connecting the metal-filled channels 216(inductor wires 1311) and capacitor elements or interdigitatedelectrodes.

At stage 1645, Parylene is applied and patterned a third time. The thirdParylene coating may have a thickness of about 5 microns and forms theflexible member 220 and an intermediate member 236 that extends betweenthe flexible member 220 and the cross bar 234 elements formed at stage1635. In the illustrated embodiment, the third Parylene layer covers thesecond photoresist coating 1636, portions of the substrate 210 and metalfilled channels 216. The third Parylene coating is applied over sectionsthat will eventually form the variable capacitor 220 and other sectionsthat will eventually form the lump inductor 1310.

At stage 1650, the backside 212 of the substrate 210 is etched, e.g.,using DRIE, and at stage 1655, the first and second photoresist layers1626, 1636 that were applied at stages 1625 and 1630 are stripped away,thereby releasing the device components.

More specifically, metal 1311 that fills the channels 216 formed throughthe entire substrate 210 form the high aspect ratio fixed inductor 1310(as further illustrated in FIG. 13), the top electrode plates orcapacitor elements 232 are joined by cross bar elements 234 and areconnected to the intermediate member 236, which extends between thecross bar elements 234 and the flexible member 220 (as furtherillustrated in FIGS. 2-4 and 13), and the bottom electrode plates formprojections or fingers 218 and corresponding channels 216 in whichcapacitor elements 232 move to vary capacitance.

It should be understood that method fabrication steps can be modified oradapted for fabrication of other structures of embodiments. The inductormay be a fixed inductor (e.g., as shown in FIG. 13), or methodembodiments can be applied to fabricate a structure having a variablecapacitor (as discussed above) and a variable inductor, e.g., as shownin FIG. 16B. The variable inductor shown in FIG. 16B may be formed bystage 1660 during which further etching 1660 of the silicon substrate210 is performed to release the metal 1311 components and form avariable inductor. Thus, embodiments can be adapted for fabrication ofvarious variable capacitor/lump inductor and variable capacitor/variableinductor configurations, and it should be understood that FIGS. 16A-Bare provided to show examples of how embodiments may be implemented.

FIG. 17 is a table summarizing expected physical, electrical andmicroelectromechanical attributes of microfabricated pressure sensorshaving a variable capacitor as shown in FIGS. 1-10 and different lumpinductors having fixed inductance as shown in FIGS. 12-15. Data in FIG.17 was derived using finite element analysis and accepted electricalmodel calculations.

Embodiments advantageously provide microfabricated pressure sensorshaving sufficiently high capacitance, inductance, resonant frequency(f_(r)), fr shift (Δf) and sensitivity (Δf/fr), and sufficiently lowresistance. For example, FIG. 17 shows that the pressure sensor 1200shown in FIG. 12 has high inductance (about 40 nh), the pressure sensor1300 shown in FIG. 13 low resistance (about 0.03 ohm) and a high Qfactor (˜600), and the pressure sensor 1400 shown in FIG. 14 has highinductance (about 145 nh) and high capacitance (about 127 pF). Thepressure sensor 1300 including the high aspect ratio inductor 1310 hasthe lowest resistance (˜0.03 ohm). Ratios of (ΔF/fr) for all threepressure sensors 1200, 1300, 1400 were determined to exceed 10⁻³,indicating that sensor embodiments would be suitable for detection by anexternal measurement device of a telemetry system.

FIG. 17 also shows that microfabricated pressure sensors constructedaccording to embodiments should have sufficient sensitivity to be ableto measure 1 mm Hg pressure changes, which correspond to a capacitancechange of about 0.4 pF, while providing for a detection range of about1-50 mmHg. FIG. 17 also shows that microfabricated pressure sensors thatinclude a variable capacitor and inductors according to embodiments areadvantageously sufficiently small in size so that they may be implantedthrough a clinical gauge needle and be implanted in various parts of aneye. For example, pressure sensors 1200 having the variable capacitor200 (FIGS. 2-4) and the inductor 1210 (FIG. 12) or the inductor 1310(FIG. 13) have dimensions of about 0.5 mm×0.5 mm 3.0 mm, and pressuresensors 1400 having the variable capacitor 200 (FIGS. 2-4) and theinductor 1410 including a rollable sheet has dimensions of about 0.5mm×0.5 mm×4.0 mm (when in a stressed or compressed configuration). Otherminimally invasive incisions may also be utilized if desired, e.g.incisions in the cornea that are smaller than about 3 mm to allowself-healing of the cornea. Further, tissue anchors may be utilized toimplant sensor embodiments without the need for sutures, e.g., asdescribed in U.S. Publication No. 2006/0247664, the contents of whichwere previously incorporated herein by reference.

FIG. 18 illustrates another embodiment of an implantable microfabricatedpressure sensor 1800 that includes a variable capacitor 1810 and a lumpinductor, e.g., the inductor 1210 shown in FIG. 12. In the illustratedembodiment, the substrate 210 is a solid substrate (that does notinclude channels 216). As discussed above, the substrate 210 may, forexample, be composed of silicon, a conductive polymer, or anothersuitable micromachinable substrate material having sufficiently highconductivity and may have a thickness of about 500 microns.

The variable capacitor 1810 is formed by one or more capacitor elements1811 disposed on the top surface 212 of the substrate 210. The capacitorelement 1811 is fixed and not movable. The variable capacitor 1810 alsoincludes one or more capacitor elements 1812 that are carried by, e.g.,embedded within, the flexible member 220. These capacitor elements 1812are movable with the flexible member 220.

FIG. 18 illustrates an embodiment in which a single capacitor element1811 or plate is positioned on the top surface 212 of the substrate 210and extends along the length of the substrate 210, and a singlecapacitor element or plate 1812 is embedded within and extends along alength of the flexible member 220. In other embodiments, each capacitorelement 1811, 1812 may be composed of multiple elements. Additionally,although the capacitor element 1812 is shown as being embedded withinthe flexible member 220, the capacitor element 1812 may also be carriedby the flexible member 220, e.g. on a top surface 221 of the middlesection 229 of the flexible member 220. FIG. 18 illustrates eachcapacitor element 1811, 1812 as a single component and the capacitorelement 1812 being embedded within the flexible member 220 for ease ofexplanation and illustration.

Further, it should be understood that during fabrication of the sensor1800, another material, such as a layer of silicon dioxide 1820 (shownin FIG. 18), may be applied on the top surface 212 of the substrate 210for purposes of providing insulation between metal elements and thesubstrate 210. Thus, the capacitor element 1811 may be disposed onsilicon dioxide 1820, but reference is made to the capacitor element1811 being on the top surface 212, which includes being directly on thetop surface 212 and a silicon dioxide layer 1820 that is applied on thetop surface 212. The capacitor 1810 is eventually sealed so that theinner space or chamber 226 is also sealed and has a fixed internal orchamber pressure (P_(c)).

The lump inductor 1210 may be configured as shown in FIG. 12 as asurface-micromachined stack of metallic layers 1211. FIG. 18 illustratesone embodiment in which the inductor 1210 includes a first or bottomlayer 1211 a and a second or top layer 1211 b separated by an insulativematerial such as Parylene. The bottom metallic layer 1211 a is disposedon the silicon dioxide 1820 layer and covered by the insulative material1212, whereas the top metallic layer 1211 b is embedded withininsulative material 1212. Referring to FIG. 19, a pressure sensor 1900may include additional layers of metallic elements, e.g., layers 1211a-e, in order to increase inductance as necessary.

In one embodiment, the bottom and top metallic layers 1211 a, 1211 b mayhave the same thickness, e.g., about 2 microns, the same width, e.g.,about 20 microns, and the same length, e.g., about 3 millimeters. Inanother embodiment, the bottom metallic layer 1211 a may be thicker thana top metallic layer 1211 b in order to increase inductance whilemaintaining flexibility of the flexible member 220 in which the topmetallic layer 1211 b is embedded. For example, the bottom layer 1211 amay have a thickness of about 2 microns, and the top layer 1211 b mayhave a thickness of about 0.5 micron.

During use, if the pressure inside of the chamber 226 exceeds theexternal fluid pressure, the flexible member 220 will retain is originalor initial shape. However, if the fluid pressure is greater than theinternal chamber 226 pressure, then the flexible member 220 will deflector be deformed by the fluid pressure, thereby moving at least the middlesection 229 and the capacitor element 1812 embedded therein closer tothe fixed capacitor element 1811 disposed on the top surface 212 of thesubstrate 210. Movement of the flexible member 220 alters thecapacitance and the resonant frequency response of a sensor circuit 1800since capacitance increases by decreasing the space between thecapacitor elements 1811, 1812, and decreases by increasing the spacebetween the capacitor elements 1811, 1812.

FIG. 20 illustrates another embodiment of a microfabricated implantablepressure sensor 2000 that is configured as shown in FIG. 18 and includesa port 2010 that is formed through the substrate 210. The capacitorelement 188 includes elements 1811 a and 1811 b within the chamber 226and on each side of the port 2010. As discussed below with reference toFIG. 21, the port 2010 may be used to evacuate materials that are usedduring fabrication of the sensor 2010.

FIG. 21 is a flow chart of a method 2100 of fabricating an embodiment ofmicromachined pressure sensor 2000 having a variable capacitor, e.g.,the variable capacitor 1810 shown in FIGS. 18-20, and a lump inductor,e.g., the inductor 1210 shown in FIGS. 12 and 18-20. It should beunderstood, however, that the method 2100 may be utilized and/or adaptedto fabricate other sensors having other variable capacitor and lumpinductor configurations. For ease of explanation, reference is made to amethod 2100 for fabricating the pressure sensor 2000 having a variablecapacitor 1810 and lump inductor 1210 shown in FIGS. 18 and 20.

At stage 2105, a substrate 210, such as a silicon wafer is provided. Thesubstrate 210 may have a thickness of about 500 microns. The top andbottom surfaces 212, 214 of the substrate 210 may be processed by knownthermal oxidation and oxide patterning methods. For example, a wet oxide(such as silicon dioxide layer 1820) having a thickness of about 1-2microns may be grown on the top and bottom surfaces 212, 214 of thesubstrate 210.

At stage 2110, a first metal layer 1211 a, 1811 a, 1811 b is depositedon the substrate 210, on the silicon dioxide layer 1820 on the topsurface 212 of the substrate 210. The first metal layer 1211 a may havea thickness of about 2 microns. Portions 1211 a of the first metal layerwill form part of the inductor 1210, and portions 1811 a, 1811 b of themetal layer will form the bottom or fixed component of the variablecapacitor 1810.

At stage 2115, a sacrificial coating of photoresist 2102 is applied(e.g., by spin coating) over portions 1811 a, 1811 b of the first metallayer, over portions of silicon dioxide 1820, and over portions of thesubstrate 210. One suitable photoresist that may be utilized withembodiments is a layer of AZ4620 type photoresist (supplied by ClariantCorp., Charlotte, N.C.). The photoresist 2102 may be hard-baked at about120° C. if applicable for smoothing of edges and degassing purposes.

At stage 2120, the top surface 212 of the silicon substrate 210 may beroughened using, e.g., XeF₂ gas-phase etching in order to promoteadhesion of Parylene to the silicon substrate 210. At stage 2125, afirst layer or coating of Parylene 1212 a is deposited and patterned. Asshown in FIG. 21, the first Parylene coating 1212 a is applied over thephotoresist 2102 and over metal layer portions 1212 a that willeventually be a part of the lump inductor 1210 and is in contact withthe silicon substrate 210 to form a seal. The first Parylene layer 1212a may have a thickness of about 2 microns.

At stage 2130, a second metal layer 1211 b is deposited over the firstParylene layer 1212 a. At stage 2135, a second Parylene layer 1212 b isdeposited and patterned. A portion of the second metal layer 1211 bforms part of the inductor 1210, and another portion 1812 of the secondmetal layer forms the capacitor element carried by and movable with theflexible member 220.

At stage 2140, after the surface micromachining process and depositionof coatings or layers, silicon material is etched away from the backside211 of the wafer 210 to create one or more through a holes, apertures orports 2010. At stage 2145, photoresist 2102 is stripped away through theport 2010 using, e.g., acetone. Backside etching may be performed using,e.g., deep reactive-ion etching (DRIE). This, in turn, releases theflexible member 220.

FIG. 22 illustrates a method 2200 that is similar to the method 2100shown in FIG. 21, except that additional backside etching is performedat stage 2205 to form a recessed cavity 2203. The recessed cavity 2203is advantageously increases the encapsulated air capacity afterpackaging of the pressure sensor.

FIG. 23 illustrates another embodiment of a pressure sensor 2300 thatincludes a variable capacitor 2310 a variable inductor 2320. In theillustrated embodiment, the substrate 210 is a solid substrate (thatdoes not include channels 216). The variable capacitor 2310 is formed byone or more capacitor elements 1811 that are on the top surface 212 ofthe substrate 210. The illustrated embodiment includes one capacitorelement 1811 that is fixed and not movable. One or more capacitorelements 1812 are carried by, e.g., embedded within, the flexible member220, and movable with the flexible member 220.

FIG. 23 illustrates an embodiment in which a single capacitor element1811 or plate is positioned on the top surface 212 of the substrate 210and extends along the length of the substrate 210, and a singlecapacitor element or plate 1812 is embedded within and extends along alength of the flexible member 220. In other embodiments, capacitorelements 1811, 1812 may be composed of multiple elements. Additionally,although the capacitor element 1812 is shown as being embedded withinthe flexible member 220, the capacitor element 1812 may also be carriedby the flexible member 220, e.g., on a top or outer surface 221 of themiddle section 229 of the flexible member 220. FIG. 18 illustrates eachcapacitor element 1811, 1812 as a single component and the capacitorelement 1812 being embedded within the flexible member 220 for ease ofexplanation. Further, as discussed above with respect to FIG. 18, thecapacitor element 1811 is described as being on the top surface 212,although it may actually be on the silicon dioxide layer 1820.

The variable inductor 2320 includes a first or bottom layer 1211 a and asecond or top layer 1211 b that is also embedded within the flexiblemember 220. For example, a first or bottom layer 1211 a may be ametallic material having a thickness of about 2 microns, and the toplayer 1211 b may be a metallic material having a thickness of about 0.5micron. The flexible member 220 may have a thickness of about 5 microns.Deformation or deflection of the flexible member 220 results in movementof portions of a variable capacitor 2310 and also movement of portionsof the variable inductor 2320.

More specifically, if the internal chamber 226 pressure exceeds externalfluid pressure, then the flexible member 220 will retain is original orinitial shape. However, if the fluid pressure is greater than theinternal chamber 226, then the flexible member 220 will deflect or bedeformed by the fluid pressure, thereby moving at least the middlesection 229 and simultaneously moving the capacitor element 1812 andinductor elements 1211 b embedded therein. The capacitor element 1812 ismoved closer to, and away from, the fixed capacitor element 1811disposed on the top surface 212 of the substrate 210 with changes influid pressure, and inductor elements 1211 b are also moved closer to,and away from, the fixed inductor elements 1211 a, therebysimultaneously altering capacitance, inductance and resonant frequencyresponse of the sensor 2300.

FIG. 24 illustrates another embodiment of a microfabricated implantablepressure sensor 2300 that is configured as shown in FIG. 23 and thatincludes a port 2010 formed through the substrate 210 and capacitorelements 1811 a and 1811 b within the chamber 226 and on each side ofthe port 2010. As discussed above with reference to FIGS. 20 and 21, theport 2010 may be used to evacuate photoresist materials that are usedduring fabrication of the sensor 2400. One manner in which the sensor2400 shown in FIG. 20 may be fabricated is illustrated in FIG. 25.

Referring to FIG. 25, a method 2500 of fabricating a micromachinedpressure sensor 2400 having a variable capacitor 2310 and a variableinductor 2320 includes providing a wafer or substrate 210, such as asilicon substrate, at stage 2505. The top and bottom surfaces of thesubstrate 210 are processed by known thermal oxidation and oxidepatterning methods. For example, a wet oxide (such as silicon dioxidelayer 1820) having a thickness of about 1-2 microns may be grown on thetop and bottom surfaces of the silicon substrate 210 and patterned.

At stage 2510, a first metal layer 1211 a, 1811 a, 1811 b is depositedon the silicon dioxide layer 1820 on the top surface 212 of thesubstrate 210. The first metal layer may have a thickness of about 2microns. Portions 1211 a of the first metal layer will form the bottomor fixed component of the inductor 1210, and portions 1811 a, 1811 b ofthe metal layer will form the bottom or fixed component of the variablecapacitor 1810.

At stage 2515, a sacrificial coating of photoresist 2102 is applied(e.g., by spin coating) over portions 1211 a, 1811 a, 1811 b of thefirst metal layer, portions of silicon dioxide 1820, and portions of thesubstrate 210. The photoresist 2102 may have a thickness of about 6microns. Thus, in the illustrated embodiment, photoresist 2102 isapplied over the entire metal layer, whereas in the embodiments shown inFIGS. 21 and 22, for example, photoresist 2102 is not applied overportions 1211 a that will form part of the inductor 2110.

At stage 2520, the top surface 212 of the silicon substrate 210 may beroughened using, e.g., XeF₂ gas-phase etching, and at stage 2525, afirst layer or coating of Parylene 1212 a is deposited and patterned. Asshown in FIG. 25, the first Parylene coating 1212 a is applied over allof the photoresist 2102 and contacts the silicon substrate 210 to form aseal. The first Parylene layer 1212 a may have a thickness of about 2microns.

At stage 2530, a second metal layer 1211 b, 1812 is deposited over thefirst Parylene layer 1212 a. A portion of the second metal layer 1211 bforms part of the variable inductor 2320, and another portion 1812 ofthe second metal layer forms part of the variable capacitor 2310, bothof which are carried by and movable with the flexible member 220.

At stage 2535, a second Parylene layer 1212 b is deposited over thesecond metal layer 1211 b, 1812 and patterned. At stage 2540, after thesurface micromachining process and deposition of coatings or layers,silicon material is etched away from the backside 211 of the wafer 210to create one or more through a holes, apertures or ports 2000, and atstage 2545, photoresist 2102 is stripped away through the port 2000using, e.g., acetone. Backside etching may be performed using, e.g.,deep reactive-ion etching (DRIE). This, in turn, releases the flexiblemember, which may then move depending on external fluid pressure. FIG.25 also illustrates an embodiment in which additional backside etchingis performed a stage 2550 to form recessed cavity 2203, which may beusefully to increase the encapsulated air capacity after packaging ofthe pressure sensor. It should be understood that method 2100 steps canbe utilized and/or adapted to fabricate variable capacitor 2310 andvariable inductor 2320 configurations, and for ease of explanation,reference is made to a method for fabricating the pressure sensor havinga variable capacitor 2310 and lump inductor 2320 as shown in FIGS. 23and 24.

Although particular embodiments have been shown and described, it shouldbe understood that the above discussion is not intended to limit thescope of these embodiments. Various changes and modifications may bemade without departing from the spirit and scope of embodiments. Forexample, pressure sensors may include a variable capacitor and a lumpinductor, or a variable capacitor and a variable inductor. Further, thedimensions and configurations of variable capacitor, lump inductor andvariable inductor components are provided as examples of how embodimentsmay be implemented, and other dimensions and configurations may beutilized to suit pressure sensing specifications and applications.Further, fabrication process parameters and steps may vary withfabrication of different capacitor and inductor configurations. Althoughembodiments are described with reference to a polymer, e.g., Parylene,flexible member and capacitor elements may be other materials, e.g., abiocompatible metal, and may be the same or different materials.Embodiments may also be utilized with variable capacitors havingcapacitor elements that are movable within channels formed in asubstrate and with variable capacitors that are implemented withoutsubstrate channels.

Although reference is made to ocular implantation of a sensor withoutsutures by delivering the sensor through a needle, it should beunderstood that other minimally invasive implantation procedures anddevices may be utilized as needed. For example, sensor devices may beimplanted through corneal or scleral incisions of a suitable size.Sensor devices may also be implanted using tissue anchors or hooks. Itshould also be understood that embodiments may be utilized in variousbiomedical applications. Although reference is made to a microfabricatedpressure sensor for passive monitoring of intraocular pressure usingtelemetry, embodiments may also be used or adapted for use in otherapplications including, but not limited to, monitoring pressure of otherbodily fluids and physiological parameters such as monitoring pressureof blood within an aneurysm, monitoring pressure of cerebrospinal fluidand monitoring pressure in other biomedical applications. Accordingly,embodiments are intended to cover alternatives, modifications, andequivalents that fall within the scope of the claims.

1. A variable capacitor of a microfabricated implantable pressure sensor, comprising: a substrate defining a plurality of channels; a flexible member having first and second edges on the substrate and a middle portion extending between the first and second edges and raised above the substrate to define a chamber between the substrate and the middle portion; and a plurality of capacitor elements extending indirectly from the flexible member, the plurality of capacitor elements being movable within respective channels with changes of fluid pressure on an outer surface of the flexible member, capacitance varying with changes in an overlapping area of the plurality of capacitor elements and the substrate.
 2. The variable capacitor of claim 1, wherein the middle portion is deformable from a relaxed shape to a deformed shape when fluid pressure on the outer surface of the middle portion exceeds an interior chamber pressure, thereby increasing the overlapping area and capacitance.
 3. The variable capacitor of claim 1, wherein the flexible member is made of a polymer or a parylene that has a Young's modulus of 1 GPa to 10 GPa.
 4. The variable capacitor of claim 1, wherein the substrate is made of silicon or a conductive polymer.
 5. The variable capacitor of claim 1, wherein the plurality of channels defines a first comb structure, the plurality of capacitor elements defining a second comb structure.
 6. The variable capacitor of claim 1 further comprising a cross bar carrying the plurality of capacitor elements and an intermediate member extending between the flexible member and the cross bar.
 7. The variable capacitor of claim 6, wherein the chamber is defined by the flexible member, the cross bar, and the intermediate member.
 8. The variable capacitor of claim 1, wherein the substrate, the flexible member and the plurality of capacitor elements are configured for intraocular implantation through a needle.
 9. The variable capacitor of claim 1 further comprising a silicon dioxide layer disposed between the substrate and the first and second edges of the flexible member. 